Relaxin is a reproductive hormone with as yet little known about its site(s) of production, chemical structure and modes of action in any species. Since porcine ovarian relaxin has been purified and the primary structure completed the differences in amino-acid sequence between the 3 microheterogeneous preparations cm-a, cm-a' and cm-B will be determined. Although these preparations are equipotent in the mouse interpubic synphysis bioassay, one is very much more potent than the other two in the in vitro rat uterine bioassay. Prorelaxin is being isolated, biosynthesized in vitro and the amino-acid sequence of the connecting peptide determined. Ovine ovarian and uterine relaxins are currently being isolated with a view to determine the amino-acid differences in relaxin(s) isolated from different tissues of a single species. Radioimmunological studies are continuing for setting up homologous assays for the pig, sheep and human and in more advanced radioimmunological terms for studying specificies of these assays in terms of the assay reported by Bryant (1972) using NIH-R-P1. BIBLIOGRAPHIC REFERENCES: Bryant, G.D. and W.A. Chamley (1976). Changes in relaxin and prolactin immunoactivities in ovine plasma following suckling. J. Repr. Fert. 46, 457-459. Bryant, G.D. and W.A. Chamley (1976). Plasma relaxin and prolactin immunoactivities in pregnancy and at parturition in the ewe. J. Repr. Fert. 48, 201-204.